As a conventional technique of restraining occurrence of high frequency wave, in other word, harmonic current in current-feed to a fusing unit of an image forming apparatus, for example, a technique is known which turns on the current-feed by 100% when a heater temperature is equal to or lower than a lower limit value, turns off the current-feed when the heater temperature is higher than an upper limit value and periodically turns on/off a sine-wave alternating current (AC) in synchronization with a zero cross of the sine-wave AC when the heater temperature falls between the upper limit value and the lower limit value.
According to the above-mentioned conventional technique, high frequency wave occurring at turning-on/off of the sine-wave AC can be reduced. However, in recent years, a more strict standard value of a harmonic current in heaters has been set and therefore, in controlling heating of the fusing unit, a technique of further restraining the harmonic current while restraining instability of the temperature of the fusing unit is in demand.
The present invention provides a technique of improving the effect of restraining the harmonic current in controlling heating of the fusing unit while restraining instability of the temperature of the fusing unit.